Red Laser is doing something similar, and being on the other side (we were developing an app that needed to use barcode scanning) I strongly disliked the idea that I have to share my sales data with them. There are also cases where agencies are doing development for their clients and have no overview over sales once they finish the app. Communicating to clients that they have to pay % of sales to some library vendor might be very hard.
Superpin is not an app, and even as a component (in difference to something like an flash/ActiveX chart component) it has a very limited market (iOS devs doing something with maps and needing to handle lot of annotations) and with sale volumes we're projecting, pricing it down to something like $19 would never pay off for the time that went into developing it.
Clustering annotations is not all too hard, but it's certainly not a trivial problem and it will take even an experienced developer (such developer should be charging at least $100/h) a day or two to do it right. If you can buy a well-tested off the shelf solution for this money, why not do that instead? This is basically the thinking behind our "big" price.